Hybrid Automatic Repeat reQuest (HARQ) with soft combining is an effective way to correct transmission errors in systems involving wireless transmission. HARQ is a variation of standard Automatic Repeat reQuest (ARQ), where an error detection code is used to detect transmission errors, and in the case errors are detected, the packet is requested for retransmission.
The basic idea of HARQ, for uplink transmission, is that after receiving data in an uplink subframe the base station, in LTE referred to as evolved NodeB (eNodeB or eNB), attempts to decode the data and then reports to the user equipment whether the decoding was successful by sending an acknowledgement (ACK) or unsuccessful by sending a negative acknowledgement (NACK). In the case of an unsuccessful decoding attempt, the user equipment thus receives a NACK in a later downlink subframe, and can retransmit the data that was not successfully received at the eNB.
HARQ with soft combining, sometimes referred to as Type II HARQ, uses Forward Error Correction to correct transmission errors from successive (re-)transmissions. The receiver stores soft copies of each failed transmission attempt. By combining the soft copies of each transmission attempt, the receiver has an increasing probability of correct decoding of the original packet. With type II HARQ, the receiver is thus able to use the cumulative energy of each transmission attempt. With plain ARQ, each retransmission attempt is decoded standalone, and thus does not have this gain.
Contention based uplink transmissions is a new technique that is being discussed in 3rd Generation Partnership Project (3GPP) for the Release 10 version of Long Term Evolution (LTE). The purpose is to reduce access delay and signalling overhead by letting user equipments transmit data on contention based (CB) grants. Contention based grants are signalled on Physical Downlink Control Channel (PDCCH) in the same way as dedicated uplink grants, but the grants are addressed to a group of users. User equipments can use the contention based grants to transmit data without prior transmission of Scheduling Request. Thus, the access time is reduced. Contention Based transmission may be feasible for users that have uplink synchronization. Contention based grants are addressed to Contention based Radio Network Temporary Identifiers, RNTIs, (CB-RNTIs). However, since many user equipments can use the same contention based grant, it is difficult to use the Hybrid Automatic Resend reQuest (HARQ) protocol to correct transmission errors, since typically errors are caused by interference from other user equipment sending data at the same time.
In the case of a decoding error in a contention based transmission scheme, the eNB will not be able to determine the identity of the transmitting user equipment. The eNB will therefore not be able to correctly combine (re-)transmissions from different user equipments transmitting of the same grant. Thus, the HARQ operation is not feasible to correct transmission errors caused by collision. Given the fixed retransmission timing used in e.g. High Speed Packet Access (HSPA) and LTE, retransmissions would only cause a new collision. However, when there is no collision, i.e. only one UE is transmitting, HARQ can effectively be used to correct transmission error, in the same way as it is used for dedicated grants.
Yet, having a retransmission scheme would significantly improve the performance in the cases where the transmissions are not successful.
In addition, a missed assignment will in general result in block errors that need to be corrected by higher-layer protocols, which in turn has a negative impact on performance in terms of throughput and latency. Also, increasing the delay may cause undesirable interactions with Transmission Control Protocol (TCP) based applications.